This invention relates to electrodepositing a hard iron alloy having a relatively low residual stress. More particularly, this invention relates to depositing a wear-resistant plate composed of an iron and cobalt alloy and comprising a high density of stress-relieving microcracks.
Hard iron plates are applied to relatively soft materials, such as aluminum, to provide wear-resistant surfaces. By hard iron, it is meant that the plate has a Vickers hardness greater than about a 400 diamond pyramid hardness (DPH). Problems have been encountered with conventional hard iron plating processes. First, plating baths, which typically contain more than 200 grams per liter (g/l) ferrous ion, must be maintained within a low, relatively narrow pH range. A pH above about 0.4 promotes oxidation of the ferrous ion and precipitation of ferric hydroxide slime and produces plates having rough or poor appearance. Hydrogen evolution increases significantly at below about 0.2 pH. Second, hard iron is plated at relatively high current densities, typically between about 10 to 20 amperes per square decimeter (A/dm.sup.2). This requires undesirably high voltages to overcome the high specific resistance of the bath. Another major problem concerns the high residual tensile stress in the product plate. For example, a 15 micrometer (.mu.m) plate may have a residual stress up to 360 megapascals (MPa). The high stress aggravates adhesion problems that result in spalling. In some cases, the high stress may be sufficient to cause cohesive failure in the substrate, which also may result in spalling. Because the residual stress increases with thickness, hard iron plates have been limited to about 30 .mu.m.
Therefore, it is an object of this invention to provide a method for electrodepositing an iron alloy onto a substrate to form a hard, wear-resistant, spall-resistant surface, which method utilizes a high-conductivity aqueous ferrous plating solution and requires relatively low voltages to obtain the high current densities necessary for hard iron plating. The method is carried out, and the solution is stable, over a relatively broad pH range above about 0.5. The solution contains cobalt that is codeposited with the iron to form a hard alloy having a relatively low residual stress.
It is also an object of this invention to provide a hard, wear-resistant plate on a substrate, which plate is formed of an iron alloy containing a minor amount of cobalt and comprises a high density of stress-relieving microcracks sufficient to reduce the residual tensile stress and thereby to improve adhesion without sacrificing wearability.
It is a further object of this invention to provide a wear-resistant surface on a substrate formed by an iron alloy plate containing up to about 6 weight percent cobalt, which plate is characterized by a Vickers hardness greater than 550 DPH and a relatively low residual tensile stress on the order of 60 MPa or less. The low stress is independent of thickness and thereby permits thicker, spall-resistant plates.